This invention relates to guardrail systems, and more particularly, to turned-down guardrail systems.
In one type of turned-down guardrail system, the guardrail barrier at the terminal slopes downwardly to ground level from normal level, which is approximately 29 inches from the ground. It is known to form the turned-down end by twisting the guardrail barrier at the end of the last support post and bending it to the ground level. The end terminal is turned-down to avoid spearing vehicles that hit it at the end rather than on the side of the barrier.
Some turned-down terminals include a drop-down barrier section supported by posts downstream of the guardrail barrier that permit the guardrail barrier to drop to the ground upon impact of the turned-down section by a vehicle. This drop-down section is designed to fall immediately from the support posts to a low level upon being impacted. The drop-down guardrail barrier is held by fastening means which release when a vehicle hits the end terminal so that the guardrail drops downwardly. In such systems, the motion of the vehicle is attenuated by hitting breakaway support posts for the guardrail.
The prior art drop-down guardrail barriers may include any of several types of fasteners to fasten the guardrails to the post. For example, in some prior art embodiments, a back-up plate is bolted to the post and mild steel straps are utilized to mount the barrier to the back-up plate. An offset block (block out or other unit) may be bolted to the post between the back-up plate and the post. Generally, a given number of posts such as seven or eight include the strap holding the guardrail barrier and an offset block whereas the guardrail barrier sections spaced further from the turned-down terminal end are bolted directly to the post.
Other prior art systems do not use metal straps but instead rely upon connecting the barrier to a back-up plate shaped complementarily such as for example with clips or bolts. The guardrail barrier may be W-beam or C-beam types or any other type of barrier. At the turned-down end, the prior art guardrail barriers are twisted flat in a counterclockwise direction and bent downwardly to the ground where they are fastened.
The prior art turned-down terminals have some disadvantages, such as: (1) vibrations due to traffic along the roadway and temperature changes cause release of some of the clip type fasteners, causing some guardrails to drop without being impacted by a vehicle; (2) under some circumstances, some vehicles impacting the guardrail terminal are launched into the air and turn over, primarily because the fasteners do not release the drop-down section, resulting in the turning over of the vehicles; and (3) some back-up plates such as those formed complementarily with W-beams do not release reliably in the drop down sections because the overlapping portions of the guardrail barrier and back-up plate hold the guardrail barrier in place.